I have a 12V charger, and a toggle switch to go switch the batteries between parallel (12V) and series (24V). The vehicle came with a charger port which is clean and I want to keep it. I handled the charging previously and always switched it to parallel before charging, but wired it so the charger port would only ever see 12V just in case.

I now have an ESC, so I want to make sure I cut the power to the unit when charging. I came up and tested the attached relay to cut the power then the charger is attached. Why doesn't it work?

I can't tell from initial tests if both batteries are charging, but the power isn't cut when plugged in, so I know it's not functioning as I expected. Thanks for the help!

Thanks for the reply. The charger definitely connects to the batteries. Before I added the relay I had the charger and the harness positives both running into the top left terminal of the switch. When you switch it to parallel they both charge in parallel. If you accidentally leave it in series, then the charger still only sees 12V, but it only charges one battery. That works very well.

With this set up all I am trying to do is make is such that once the charger is causing current to travel through it's wires, then the relay would trip and cut power to the harness.

I'm not sure how much current is required to trigger these relays, but I assume the charger outputs enough for that.

If i connected the other coil terminal to the charger ground (instead) wouldn't that just short the charger?

I personally do not advocate charging multiple batteries in parallel with a single charger. That being said...

I'd take a slightly different approach. Rather than use your charger to close the relay (which isn't easily do-able with that setup), use your series/parallel switch to close the relay in series mode. Think of it more like a Run/Charge switch

In my diagram the charger contacts the batteries through the switch. I'm not sure how else to explain this.

I already have it set up as a run/charge switch. I am trying to add the functionality of automatically cutting power to the harness (ESC) when charging.

Thanks for your input. I have the following questions/concerns about your diagram:- There is risk of the charger seeing 24V.- There is a constant drain through the relay when not in charge setting.- Would the battery short (in a damaging way) when through the coil when batteries are in series?

and by 'working', I mean potentially not fully charging the battery on the left. Also, if the switch gets moved to series while your charger is plugged in, that relay will get 24V. So will your charger but it should fault out and stop charging.

*Charging each battery individually is best and each battery will charge with 0 load.*The relay takes the batteries out of series and breaks any circuit to your ESC when the switch is off. Your ESC sees 0V.*Flipping the switch will put the batteries in series and disconnects the left battery from the charger. Right battery will remain connected but poses no electrical threat other than live 12v at the charge port pins 1 & 2. Your design would have live 24v I believe, so it's still an improvement.*Yes, the battery on the right will have the burden of the relay (250 mA) during drive time/when the switch is on but it's pretty insignificant and likely will not affect drive time unless allowed to sit idle for extended periods with the switch on. For this reason, I usually use a key switch. Kids aren't dumb... they know that when mom and dad park the car they turn the key off. They will usually do the same.

Now, you could take this concept and apply it to your design and use one of the chargers to trip the relay when you plug it in if you want. That battery will always take longer to charge but the big thing is you should really charge them separately.

Bottom line, your current set up is not working because you are not using the relay properly.There is a FAQ about relays in the 'How Stuff Works' sectionviewtopic.php?f=33&t=104

So out of pure curiosity, I went in the engineering lab and mocked up your diagram, and I'll be danged, your charger is getting to your battery. I never thought much about using a relay in this fashion, mainly because it doesn't make the relay work, but yes, you are correct, you can charge a battery like that. I would have to imagine a 2A charge rate would make that coil rather warm though.

Everything in your diagram works except the relay itself. Problem is, in order to make the relay do what you want, none of the rest can be the way it is.

Thanks for the replies. I've been away for a few days so sorry to drop my end of the conversation.

I've definitely learned some from your link, thanks. It's still strange to me that there isn't a strong enough magnetic field being generated there. An electromagnet ONLY needs current to pass through it. There is current passing through it...so why wouldn't it trigger? This is driving me crazy. Anyway, not your worry if you don't car. It's just been something gnawing at me.

I'm no expert by any means but I suspect it has something to do with the fact we're charging a battery instead of running a load. I'm sure if you put it in series with a motor it would work as you intended but I think charging may be a different beast. Who knows.

I think all you nees to do to make it work is to make an additional connection from the charger port + to the battery +. Then disconnect the relay coil from the battery + and connect it to ground. That should throw the relay when you plug in the charger and disconnect the battery from the harness...